I appreciate all the really good comments here, fantastic knowledge base.

Are they soldered on?
I am guessing, NO. remember that is guessing. In a factory setting probably easier
to drill and tap, then screw it on.

I want to make up some iron sighted guns, and all this could be helpful.
One thing to consider is how much iron sights now cost, like an arm and
a leg, yet they used to come standard. It might be good to buy one of
these iron sighted barrels, unscrew it from the barrel, plug the holes
and now have a set of sights at a pretty low cost. You could then make
up some barrel bands, that will clamp onto the barrel, mount the sights
on the barrel bands. Hey, you got to waste you life doing something,
why not make up some iron sighted barrels?